Although the term "energy harvesting" has been gaining popularity lately, the amount of energy harvestable from our immediate surroundings is not large. In fact, most energy harvesting experiments are dealing with less than 1mW of energy. While few applications benefit from such small amounts of energy, we are beginning to find out that energy harvesting can compensate for the weakness in wireless sensor networks (WSN) when used as their energy source.
This article will introduce Murata's energy harvesting-related efforts.
What is Energy Harvesting?
Energy harvesting is a series of processes including collecting energy around us, converting it to electricity and operating small equipment with it. We all know that we can generate a lot of electric power if we use fuel, but that is not called energy harvesting. Energy harvesting does not include electric energy generated from conscious human effort either. The key idea here is being able to generate electricity "unconsciously."
Careless harvesting may end up overburdening an energy source by trying to get as much energy as possible. With all honesty, it is impossible to generate a large amount of energy through energy harvesting anyway. Such attempts would result in creating devices unrealistically large or costly, defeating the purpose.
Therefore, energy harvesting only makes sense when it is done through the effective and thrifty utilization of small amounts of harvested energy.
Energy Around Us
Figure 2 lists kinds of energy we can find around us. The unit for energy is joule (J) and 1J is 1 watt second. This list reveals that the level of harvestable energy around us is very small. In comparison, the level of energy used by equipment around us is quite high. We often receive inquiries about the plausibility of charging mobile phones with energy harvesting.
It should be evident from the right column of Fig. 2 that it will be very difficult. We are also often asked if it is possible for us humans to generate enough energy to supply household energy. Judging for the calories we consume from food, it is easy to surmise that the energy we can possibly generate is also small. Furthermore, three quarters of the calories are used by our basal metabolism. The amount of energy a person may generate per day, without conscious effort, is less than 1mJ. The key mission of energy harvesting is the effective utilization of such small amounts of energy.
Fig. 2 Energy comparison